γ,δ-Unsaturated carboxylic acids have in the molecule two reactive sites, an olefin moiety and a carboxyl group. They are useful as polymerizable monomers in polymer manufacture or as intermediates for the synthesis of pharmaceutical and agricultural chemicals. With respect to 2,2-dimethyl-4-pentenoic acid, for example, U.S. Pat. No. 5,534,562 discloses its use in a primer composition for dental material bonding. Also, JP-B 6-43414 discloses its use as raw material from which pharmaceutical intermediates are prepared.
For preparation of γ,δ-unsaturated carboxylic acids and esters thereof, several processes have been reported, including (a) allylation at α-carbon of carboxylic acids, (b) oxidation of γ,δ-unsaturated aldehydes, and (c) Claisen rearrangement.
Process (a) includes (a-1) where a base acts on a carboxylic acid or ester thereof to generate carbanion at α-position, with which an allylating agent is reacted; (a-2) where a malonic acid ester is reacted with an allylating agent in the presence of a base and a palladium catalyst, followed by decarboxylation; (a-3) where a metal acts on α-halogenated carboxylic acid ester, followed by reaction with an allylating agent; and (a-4) where using a lithium reagent and trialkylchlorosilane, a carboxylic acid ester is converted to a silyl ketene acetal, which is reacted with an allylating agent in the presence of a palladium catalyst. In all these processes, the base or metal must be used in excess of the stoichiometry. This gives rise to drawbacks including a reduced yield per unit reactor volume and the formation of much salt to be discarded.
Qingdao Haiyang Daxue Xuebao, 1999, Vol. 29, pp. 319-320, reports successful results of producing 2,2-dimethyl-4-pentenoic acid in high yields by process (b) using silver oxide as an oxidizing agent. USSR Patent No. 1,397,428 discloses a method of making 2,2-dimethyl-4-pentenoic acid by process (b) using cobalt acetate as a catalyst and molecular oxygen as an oxidizing agent in methanol. However, the synthesis of γ,δ-unsaturated aldehyde used as the starting material is not always satisfactory in yield, cost, reaction time and the like.
By contrast, preparation of γ,δ-unsaturated carboxylic acids and esters thereof by (c) Claisen rearrangement is ideal in that rearrangement reaction per se forms no waste products. See Trost and Fleming Ed., Comprehensive Organic Synthesis, First Edition, Pergamon Press, 1991, pp. 827-873. Problems arise in that ketene acetals used as the starting materials in rearrangement reaction are produced by transesterification (Johnson-Claisen rearrangement) between ortho-ester and allyl alcohol at high temperature, or deprotonation-silylation (Ireland-Claisen rearrangement) of carboxylic acid allyl ester. The former uses a high reaction temperature and lacks selectivity. The latter requires at least one equivalent of the deprotonation agent, from which a large amount of salt is formed. Besides, it was reported to perform Claisen rearrangement by subjecting zinc to act on α-bromocarboxylic acid allyl ester. This method must use an excess amount of zinc powder, undesirably producing a large amount of waste.
JP-A 9-202791 describes that when allyl acrylate is hydrosilylated in the presence of a platinum catalyst, a γ,δ-unsaturated carboxylic acid and silyl ester thereof are formed as by-products through Claisen rearrangement. In this process, formation of γ,δ-unsaturated carboxylic acids takes place as side reaction and only in low yields.
Although γ,δ-unsaturated carboxylic acid derivatives are useful compounds, their preparation process is limited as discussed above. There exists a need for a simple process for their preparation in high yields.
Meanwhile, organosilicon compounds having a carboxyl group are useful as silane coupling agents, precursors raw materials for various modified silicone fluids, and raw materials for polycondensation polymers such as polyamides and polyesters. For their preparation, JP-A 2001-158791 and JP-A 11-193291 disclose methods of preparing siloxanes having carboxylic acid and carboxylic acid ester moieties. In either case, hydrosilylation is utilized to form a silicon-carbon bond. Due to mild reaction, hydrosilylation is effective for the synthesis of silicon compounds having a carboxyl group or precursor thereof. However, there are commercially available few unsaturated carboxylic acids and equivalents to be used as the starting material. Then, the type of carboxyl group-containing organosilicon compounds that can be produced using such starting materials is also limited. It is desired to solve these unsatisfactory problems.